A combustion-sustaining gas and a fluid combustible (fuel) are mixed and fed under pressure to a combustion chamber where they are burned. Since combustion itself is a dynamic process it is impossible to make the combustion in the chamber perfectly stable even under ideal conditions in which a molecularly homogeneous mixture of the combustion-sustaining gas and the gaseous or finely divided fluid combustible is used.
Recirculation of the exhaust gas has been attempted in recent times in order to increase the efficiency of the system and decrease pollution created by the combustion. There are two main types of such recirculation or feedback: internal feedback which is effected purely by the shape of the combustion chamber and/or the flow of the gases or fluids in the chamber, and external feedback wherein exhaust gases drawn off the top or downstream end of the combustion chamber or from conduits connected to said combustion chamber are mixed with air and introduced under pressure back into the combustion chamber.
The main intent in such recirculation systems is to dilute the oxygen content of the air, thereby lowering the partial pressure or concentration of oxygen in the mixture of combustion-sustaining gases.
For a given mass flow it is therefore possible to greatly reduce the excess quantities of air, so as to decrease the production of nitrogen oxides and decrease the amount of soot formed.
External recirculation of combustion or exhaust gases is made through a conduit or pipe which extends from the upper end of the combustion chamber to the input of the blower that supplied air to the burner of the combustion chamber. It has been discovered that the feedback circuit causes oscillations which can build up and extinguish the flame in the combustion chamber. These oscillations are set off in the chamber and propagate through the circulation circuit. If the triggering pressure waves are at a given frequency for the system the effect is cumulative and regenerative.
For this reason external feedback has been little used, as outside of laboratory conditions it has been found to be impossible to keep the system under control. The flame is almost invariably very unstable, as the combustion produces a noise that can resonate in the feedback circuit and, when the resonance has a wavelength which is equal to or a multiple of the length of the feedback circuit it is possible for this pressure wave to amplify itself and extinguish the flame. It is noted that the noise produced at the flame is an undesirable phenomenon because it indicates a poor mixture between the combustible and the combustion-sustaining gas.